Flame sensors are used to sense the presence or absence of a flame in a heater or burner, for example, or other apparatus. The heater or burner may be used to heat water or ambient air and the fuel used may be one of several different types.
In the event the flame is extinguished, although not deliberately so, the sensor is adapted to sense the absence of the flame. The flame can be extinguished, for example, by fuel starvation or other malfunction. After sensing the extinguishing of the flame, the sensor or its related circuitry will send an alarm signal to a microcontroller. The microcontroller will take appropriate action such as shutting down the heater or burner by terminating fuel flow. In such a manner, serious safety problems such as continued fuel flow into a hot burner without a flame being present for combusting the fuel are avoided.
However, it is inconvenient to terminate the fuel flow if the flame is present and the burner is working properly. The termination of the fuel flow causes termination of the operation of the burner or heater unintendedly if the flame sensor sends an incorrect signal to the control panel. The present invention has as an object the avoidance of inadvertent burner shutdown and, as well, the avoidance of burner operation when the flame is extinguished.
One reason for unintended burner shutdown is signal contamination of the signal from the flame sensor, Since the power of the signal previously sent to the amplifier is quite small, in the range of 50 mv to 200 mv, and since the amplifier was located some distance from the sensor, any noise caused by common mode radiation or other RF signals could disrupt the integrity of the signal being passed to the amplifier by the sensor. This causes incorrect information to be read by the microcontroller with the result that the heater could be inadvertently shut down or, alternatively, the heater may continue to run in a flame out condition. Both scenarios are not desirable.
In our U.S. patent application Ser. No. 09/579,444 filed May 26, 2000, the contents of which are incorporated herein by reference, there is disclosed a circuit for a flame sensor which utilises an amplifier and rectifier circuit in which full amplification of the pulsed signal leaving the amplifier does not take place due to a feedback loop between the output of the amplifier and the inverting input of the amplifier. This leads to a decreased reading sensitivity of the pulsed signal generated by the photodiode of the flame sensor.
To sense the presence or absence of a flame in a burner, a photo diode is generally used. As the flame flickers in the burner during operation, the photo diode will sense the flickering or change of light in the flame and generate a signal to indicate the presence of a flame. The components of the heater, however, generates noise and this noise can produce a signal which is similar to the signal generated by the photo diode when a flame is present. Thus, the flame sensor circuit may incorrectly signal that a flame is present.